In recent years, in response to social demands and movements arising from energy and environmental issues, a fuel cell capable of being operated at normal temperature to obtain high power density has been attracting attention as a power source for electric vehicles and as a stationary power source. A fuel cell is a clean power generation system wherein water is principally generated by an electrode reaction and there are almost no adverse impacts on the global environment. In particular, a polymer electrolyte fuel cell (PEFC) is anticipated to be a power source for electric vehicles because the PEFC is operated at a relatively low temperature. Generally, the polymer electrolyte fuel cell has a structure wherein an electrolyte membrane-electrode assembly (MEA) is interposed by separators. The electrolyte membrane-electrode assembly is configured such that a polymer electrolyte membrane is interposed by a pair of electrode catalyst layers and a pair of gas diffusion electrodes (gas diffusion layers; GDLs).
In the polymer electrolyte fuel cell having the electrolyte membrane-electrode assembly as described above, an electrode reaction represented by the following reaction proceeds according to polarities of both electrodes (cathode and anode) interposing the solid polymer electrolyte membrane to yield electrical energy. First, hydrogen contained in a fuel gas supplied to the anode (negative electrode) side is oxidized by a catalyst component, to form a proton and an electron (2H2→4H++4e−: Reaction 1). Next, the produced proton reaches a cathode (positive electrode)-side electrode catalyst layer through a solid polymer electrolyte contained in the electrode catalyst layer and the solid polymer electrolyte membrane contacting the electrode catalyst layer. In addition, the electron produced in the anode-side electrode catalyst layer reaches the cathode-side electrode catalyst layer through a conductive carrier constituting the electrode catalyst layer, a gas diffusion layer contacting the opposite side of the electrode catalyst layer to the solid polymer electrolyte membrane, a separator, and an external circuit. Then, the proton and the electron, which have reached the cathode-side electrode catalyst layer, react with oxygen contained in an oxidant gas supplied to the cathode side, to produce water (O2+4H++4e−→2H2O: Reaction 2). In the fuel cell, electricity can be taken out to the outside through the above-described electrochemical reaction.
In order to improve power generation performance, improvement in activity and durability (activity after a durability test) of a catalyst particle in the electrode catalyst layer is an important key. Conventionally, from the viewpoint of the improvement in the activity and durability, it has been necessary to use platinum as a catalyst component of electrode catalyst. However, since the platinum is very expensive and is also a rare metal as a resource, there has been a need to develop a platinum alloy-based catalyst by reducing a content of platinum occupied in the catalyst particle while maintaining activity or durability.
For example, Patent Literature 1 discloses a catalyst containing a platinum-metal alloy having a face-centered tetragonal structure and showing a broad peak or a peak having two split tips at a 20-value of about 65 to 75° in an XRD pattern of the platinum-metal alloy. According to Patent Literature 1, since the platinum-metal alloy having the face-centered tetragonal structure is stable in structure, the durability is excellent.